In recent years, the field of surgical medicine has taken profound strides in the replacement of diseased or damaged natural organs with healthy organs from human donors. Typically, the donor organs are obtained as a result of the accidential death of the organ donor. These transplants have become relatively commonplace and liver and heart transplants are a viable medical option in the treatment of liver and heart disease. Transplant surgery is complicated and expensive, but it certainly has passed out of the purely experimental stage of development into a place of an acceptable medical alternative to other forms of treatment. In many cases, particularly where a weak and failing heart is involved, a heart transplant is the only meaningful way to extend the life expendency of the recipient from little or no life expendency to an additional life expendency which is measured in terms of years.
As advanced as organ transplant surgery has become, particularly heart transplant surgery a fundamental limiting problem with such surgery is always present. In every heart transplant, a donor must die that the recipient can live. There are a limited number of donor hearts available at any given time. In most cases, the proposed recipient is in critical condition; and a relatively narrow time frame, exists during which a successful heart transplant can be effected. Frequently, no donor heart is available during this time frame and the potential recipient dies of heart failure.
Temporary life support for a patient whose heart has failed is available from ex-vivo heart-lung machines. Such machines remove the blood from the body of the patient and provide the necessary carbon dioxide/oxygen exchange prior to returning the oxygen laden blood to the patient. Substantial trauma results in such machines which produce thrombosis of the blood cells. The quality of life experienced by a patient on such a machine is certainly one which would necessarily be classified as poor and cannot be sustained indefinitely.
In an effort to fill the void existing as a result of the limited availability of natural human hearts for transplant and the serious shortcomings of heart/lung machines, development of total artificial hearts (TAH) has been undertaken. During the past decade, one design of such a total artificial heart which has met with a relatively high degree of success is a pneumatic drive "diaphragm" artificial heart design. Currently, one of the more popular hearts of this design is known as the "Jarvik 7" heart. Such diaphragm type artificial hearts have separate right and left ventricles made of body compatible materials of a generally spherical shape. A thin substantially semi-spherical diaphragm divides each of the ventricles of the housing into two compartments. One of the compartments in each ventricle has a port in it for attachment to pneumatic drive lines. The other compartment in each ventricle has blood inlet and blood outlet ports fitted with mechanical disk valves. Segmented polyurethane is used to fabricate the surfaces of such artificial hearts which come into contact with blood and other body tissues. Two pneumatic drive lines are connected to the ports in the respective ventricles and extend to a pneumatic drive unit. This drive unit delivers an alternating flow of positive and negative air pressure through the drive lines to the ventricles to cause alternate pushing and pulling (back and forth) movement from the diaphragm to alternately force blood out of the second compartment of one ventricle while drawing blood into the second compartment of the other ventricle and vice versa.
Two primary problems are present in such diaphragm-type artificial hearts. First, the stroke volume of the ventricle is directly related to the diameter of the diaphragm. For any given diaphragm material, the range of movement of the diaphragm between diastole and systole is directly dependent on the diameter of the diaphragm. This is referred to as "travel distance". The travel distance cannot be changed without an appropriate change in the diaphragm diameter. Consequently, the stroke volume from such an artificial heart ventricle is severely limited by the diaphragm diameter; and a small reduction in the diaphragm diameter produces a markedly reduced stroke volume.
A second limitation of diaphragm type artificial heart ventricles results from the generally spherical shape of the ventricles which form the complete artificial heart. This creates a relatively small area of contact between the two ventricles after implantation. As a result, the limited space of the thoracic cavity is inefficiently used; and, particularly for smaller patients (such as most women and men of small physical size), this results in a reduction in the stroke volume when the ventricle size is decreased for better anatomical fit.
The diaphragm type TAH's currently being used for clinical trials is the Jarvik 7. This TAH typically measures 9 centimeters from the atrial connectors to the top of the ventricles; and the combined width of both ventricles is nearly 15 centimeters, with a maximum stroke volume of approximately 100 ml. The "mini-Jarvik" is 8 centimeters high and 14 centimeters wide and has a stroke volume of 60 ml. Current clinical experience suggest that the 100 ml. stroke volume may not be adequate for a person having a thoracic cavity and body large enough to accommodate the Jarvik 7 TAH. To compensate for this relatively low stroke volume, the number of beats per minute (pulse rate) of Jarvik 7 TAH devices typically is somewhere between 100 and 130 beats per minute. This is a substantially higher pulse rate than the normal pulse rate provided by a natural human heat. The design of a Jarvik 7 diaphragm type heart, however, is such that a larger volume cannot be obtained due to the limitations of the space available in the thoracic cavity.
Another clear disadvantage of current diaphragm type TAH devices is that such devices require two relatively large diameter pneumatic drive lines extending from a pumping unit external of the body of the artificial heart within the body. These drive lines inherently increase the danger of infection where they pass through the body and also result in depression of the lungs and other vital organs where the lines connect into the artificial heart device. In addition, significant restriction of movement of the patient is involved because of the external driving machine.
It is desirable to provide a total automatic heart (TAH) which overcomes the limitations of the prior art TAH devices discussed above. More specifically, it is desirable to provide a TAH device which more closely approximates the size and shape of a natural human heart and which has a substantially higher stroke volume in proportion to the size of the ventricles than is present in known prior art TAH devices. It also is desirable to provide a TAH device which reduces the potential for thrombosis and hemolysis.